Monoethylene glycol (MEG) and monopropylene glycol (MPG) are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers such as polyethylene terephthalate (PET).
Said glycols are currently made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, generally produced from fossil fuels.
In recent years increased efforts have been focussed on reducing the reliance on fossil sources as a primary resource for the provision of fuels and commodity chemicals. Carbohydrates and related ‘biomass’ are seen as key renewable resources in the efforts to provide new fuels and alternative routes to desirable chemicals.
In particular, certain carbohydrates can be reacted with hydrogen in the presence of a catalyst system to generate polyols and sugar alcohols. Current methods for the conversion of saccharides to glycols revolve around a hydrogenation/hydrogenolysis process as described in Angew. Chem. Int. Ed. 2008, 47, 8510-8513. Development of this technology has been on-going.
A preferred methodology for a commercial scale process would be to use continuous flow technology, wherein feed is continuously provided to a reactor and product is continuously removed therefrom. By maintaining the flow of feed and the removal of product at the same levels, the reactor content remains at a more or less constant volume. Continuous flow processes for the production of glycols from saccharide feedstock have been described in US20110313212, CN102675045, CN102643165, WO2013015955 and CN103731258.
Reported processes for the conversion of saccharides to glycols generally require two catalytic species in order to perform the hydrogenation/hydrogenolysis process. The first catalytic species catalyses the hydrogenolysis reaction, which is postulated to have a retro-aldol mechanism, and the second catalytic species is present for the hydrogenation reaction.
The catalytic species used for the hydrogenation reactions tend to be heterogeneous. However, the catalytic species suitable for the retro-aldol reactions are generally homogeneous in the reaction mixture.
The use of a homogeneous tungsten-containing species as the first ‘retro-aldol’ catalytic species has been reported widely, for example in US20110312487; US 201103046419; Angew. Chem. Int. Ed. 2012, 51, 3249-3253; AIChE Journal, 2014, 60 (11), pp. 3804-3813; and WO2016114661. The use of a sodium metatungstate-containing species as the retro-aldol catalytic species is disclosed in co-pending application EP 15195495.5.
The homogeneous tungsten-based catalysts typically used in a saccharides to glycols process may be susceptible to conversion to undesirable products, for example by reduction and precipitation of the metal (tungsten). Precipitated solids in a reactor system can lead to blocked lines and clogging as well as undesirable chemical and/or physical reactions of the tungsten metal with other species present (e.g. catalyst poisoning).
It is clearly desirable to maximise the yields of MEG and MPG in saccharides to glycols processes and to deliver a process that can be carried out in a commercially viable manner. The market for MEG is generally more valuable than that for MPG, so a process particularly selective toward MEG would be advantageous.
Saccharide-containing feed streams are subject to degradation when held at the elevated temperatures required for their conversion to glycols for any significant period of time. Saccharide degradation includes conversion to less useful saccharides (e.g. glucose conversion to fructose) as well as other undesirable non-saccharide by-products. Degradation and/or conversion may also occur at lower temperatures which are experienced during the heating up (including mixing) of the feedstock to the reaction temperature. Saccharide degradation is undesired, as product yields are lower and fouling might occur, in addition to separation of desired product from degraded products and waste handling, including waste water treatment. Saccharide degradation has typically been reduced by limiting the residence time of the saccharide-containing feed at elevated temperatures before the feed is introduced into the reactor and equalizes in temperature and concentration with the reactor liquid content, i.e. during the time present in feed lines and in the reactor before mixing is complete and reaction occurs. Limitation of saccharide degradation to 5% or less is difficult, as reaction times for such degradation at temperatures higher than about 160° C. are in the order of a few seconds. Liquid handling within such time frames are difficult in industrial practice and, therefore, highly undesirable.
One undesirable side-reaction of saccharides at high temperature, in the conversion of saccharides to glycols, involves the conversion of glucose to fructose. It is postulated that when fructose undergoes hydrogenolysis in a retro-aldol process, C3 fragments are produced, increasing the relative amount of MPG formed compared with MEG.
The use of a buffer in a process for the conversion of saccharides to glycols has been described in co-pending application EP15184082.4. Such buffers are used to maintain the pH in the reactor within a preferred range and are typically alkali metal, preferably sodium, containing salts.
It is desirable to provide an improved process for the conversion of saccharides to glycols in which the yield of glycols and, preferably, the yield of MEG is maximised.